1. Field of the Invention
The present invention relates to a process of producing a rigid polyurethane foam having flame resistance and more particularly, it relates to a process for producing a rigid polyurethane foam having a high flame resistance and unaccompanied by a reduction in the properties of the polyurethane foam or by the formation of scorching. The invention further relates to a flame resistant rigid polyurethane produced by such a process. The term "rigid polyurethane foam" used herein refers to a foam which loses its original rigidity when the foam is compressed to a compression ratio more than 50% due to the rupture of cells. The rigidity of a given foam is generally indicated by a rigidity at a 10% compression ratio.
2. Description of the Prior Art
Polyurethane foams have hitherto been used in a large number of fields due to their specific properties but have a serious disadvantage in that polyurethane foams are easily flammable.
Recently, however, regulations on the extent for rendering polyurethane materials flame retardant or flame resistant have become more and more severe, and this property is particularly important in the application thereof in the fields of automobiles, railway vehicles, aircraft, etc.
The rendering of flame-retarding properties or flame resistance to polyurethane foams is an important problem as described above, but it has not yet been solved by conventional techniques. The methods which have been industrially employed at present still have a large number of difficulties.
Some techniques for rendering polyurethane foams flame retardant are described in the specifications of, e.g., Japanese Pat. Publication Nos. 1750/'63; 9197/'70, 349/'64; 4846/'64; 8696/'64; 26335/'71; 21358/'69; 13037/'66; 2269/'71; Belgian Pat. No. 723,246; U.S. Pat. Nos. 3,262,894; 3,309,342; 3,385,900; 3,402,132; 3,407,150; 3,455,850; 3,530,205; and 3,535,406; British Pat. Nos. 918,636; 954,712; 999,588; and 1,043,832; etc., but these techniques are insufficient for satisfying the necessary requirements.
For example, Japanese Pat. Publn Nos. 1750/'63; 9197/'70; etc., disclose the use of phosphorus-containing compounds or halogen-containing compounds as flame retarders for polyurethane foams but, in such known techniques the phosphorus-containing compound must be added to such an extent that the content of phosphorus in the foam is higher than 1% or the flame retarder must be blended so that the content thereof is 5 to 30% by weight of the polyurethane foam. As a result of using such a large amount of flame retarder, the properties of the polyurethane foam are degraded, for example, the hardness is reduced, the strength is reduced, etc. Furthermore, when the polyurethane material is foamed excessively, the so-called scorching phenomenon occurs which lowers the commercial value of the article. Moreover, since in such cases the concentration of the flame retarder is high, the flame retarder tends to volatilize away due to the action of heat or with the passage of time, which results in a reduction in the flame-proofing or flame-resisting effects. In order to prevent the formation of such scorching, the content of the flame retarder must be reduced to as small an amount as possible. Therefore, the development of a foaming technique which can render polyurethane foams flame retardant using as little an amount of flame retarder as is possible or without using any flame retarder has been highly desired.
As described above, an important problem has been to improve the flame resistance of the polyurethane foam by blending a halogen-containing or phosphorus-containing compound in an amount as small as possible.
The extent of the reduction of the amount of retarder depends upon the extent of flame resistance required in the polyurethane foam and the properties of the flame retarder used, but it is preferred that the amount be as small as possible and further it is most preferred that a flame retarder not be used at all.